1. Field of the Invention
This invention relates to a focus detecting optical system used in a digital camera or a silver halide film single-lens reflex camera, and in particular, to a focus detecting optical system in which the pupil of a photographic lens is divided into two areas and two secondary object images are formed by light beams passing through the areas so that the relationship between the relative positions of the secondary object images is detected to thereby find the focus position of an imaging lens, and a camera provided with the focus detecting optical system.
2. Description of Related Art
There are conventional focus detecting optical systems in which the primary image formed by a photographic lens is re-formed into two images on a pair of image sensors through a re-imaging optical system including a condenser lens and a pair of re-imaging lenses so that the light intensity distributions of the two images are compared to make a correlation calculation and thereby a distance between the two images is found to obtain the amount of defocus. For example, when the distance between the two images derived from the calculation is represented by Y0 and the distance between the two images at a given point is represented by Y1, there is a correlation between a difference δ=Y1−Y0 and the amount of defocus, and thus focus detection can be made by finding the difference δ.
In such focus detecting optical systems, in order to improve range measuring accuracy and to correct aberration, various proposals have been made as disclosed, for example, in Japanese Patent Kokai Nos. Sho 60-32012, Sho 62-25715, Sho 62-69217 (Japanese Patent Publication No. Hei 7-31300), Sho 60-32013, and Sho 62-79407.
In the focus detecting optical system disclosed in each of Kokai Nos. Sho 60-32012 and Sho 62-25715, in order to correct distortion, at least one surface of the condenser lens is configured as an aspherical surface, such as a hyperboloid of revolution or an ellipsoid.
In the focus detecting optical system disclosed in Kokai No. Sho 62-69217 (Publication No. Hei 7-31300), in order to correct distortion and chromatic aberration, entrance and exit surfaces of each of the re-imaging lenses are decentered. In the focus detecting optical system disclosed in Kokai No. Sho 60-32013, in order to correct distortion and chromatic aberration, the centers of stops and the R surfaces of the re-imaging lenses constructed with plano-convex lenses are decentered. In the focus detecting optical system disclosed in Kokai Sho 62-79407, in order to correct distortion and chromatic aberration, one surface of each of the re-imaging lenses is configured as a spherical surface and the other surface is configured as an inclined flat surface with a prism function.
By the recent development in semiconductor technology, great strides has been made in compactness and hyperfine structure of a solid-state image sensor used in a digital camera. Even in a single-lens reflex digital camera using the solid-state image sensor, there is a strong demand for compactness of the solid-state image sensor.
Here, in order to downsize the camera of this type, when an image circle is made smaller than in a conventional camera of 135F or APS size, for example, a camera that has the image circle of about a half, as compared with the case of 135F, is assumed. When the focus detecting optical system corresponding to the conventional camera of 135F is used, as it is, for any optical system including an image sensor for detection as the focus detecting optical system in an attempt to print a photographed image of the same size, an enlarging magnification is roughly doubled. When this photograph is viewed under a constant observing condition, the radius of an allowable circle of confusion must be thought of as about a half, as compared with the conventional case of 135F. Hence, although the radius of the allowable circle of confusion is reduced, a pixel pitch is relatively increased and thus detection accuracy is degraded. In this case, in order to prevent the degradation of detection accuracy, it is necessary to reduce the pixel pitch of the image sensor for detection or increase the imaging magnification of the focus detecting optical system. Consequently, in order to downsize the camera of this type, the focus detecting optical system corresponding to the conventional camera of 135F or APS size cannot be adopted.
Furthermore, when the pixel pitch of the image sensor for detection is decreased or the imaging magnification of the focus detecting optical system is increased, correction for distortion or chromatic aberration of the same extent as in the conventional focus detecting optical system may cause the error of range measurement, and more complete correction than in the conventional focus detecting optical system becomes necessary.
Therefore, in order to downsize the camera of the type, it is desired to provide a focus detecting optical system which has a high degree of focus detecting accuracy corresponding to a small image circle and in which aberration is completely corrected.
Although the conventional focus detecting optical system is proposed on the premise that the camera of 135F or APS size is used, no one has yet been able to produce a focus detecting optical system, based upon the premise that the camera of the type is downsized, in which aberration is corrected in accordance with this downsizing and range measuring accuracy is attained, and a camera provided with this focus detecting optical system.